Acyclovir and zidovudine are the two most widely used antiviral drugs. Acyclovir is efficacious against all infections caused by herpes simplex virus, but treatment must start early to be effective. Herpes zoster virus is less susceptible to acyclovir, but high doses shorten the duration of skin lesions, although the effect on post-herpetic neuralgia is uncertain. Zidovudine diminishes short-term mortality in patients with HIV infection and serious opportunistic infections. In those patients, the average increase in life expectancy is about one year. Because of myelotoxicity, frequent monitoring of blood counts is necessary. Recent results in patients who have few or no symptoms of HIV infection indicate that the drug decreases the chance of progressing to AIDS. Therefore, indications for treatment now include asymptomatic patients with unfavourable laboratory parameters.